1. Field of the Invention
The present invention relates to systems and methods for detecting and locating ground faults in electrical distribution systems. More particularly, the present invention relates to systems and methods for detecting and locating ground faults in ungrounded AC or DC system without de-energizing the circuit.
2. Background Art
An electric circuit provides a path for electric current to travel that is typically composed of conductors, conducting devices, and a source of electromotive force that drives the current around the circuit. Current flows in an electric circuit in accordance with several definitive laws, including Ohm's law, which provides that the amount of current flowing in a circuit made up of pure resistances is directly proportional to the electromotive force impressed on the circuit and inversely proportional to the total resistance of the circuit. Ohm's law applies to circuits for both direct current (“DC”) and alternating current (“AC”), but additional principles, such as Thevinin's theorem, must be invoked for the analysis of complex network circuits and for AC circuits also involving inductances and capacitances.
An electric circuit may include an intentional electrical connection from a conductor to an electrical ground termed “a grounded system” or a circuit may be intentionally left ungrounded. Such ungrounded systems typically contain a ground detection device that is intentionally grounded through a resistance in the detector. This ground path only serves to provide a reference to ground from either the positive conductor or the negative conductor and should not be confused with a grounded system. In an industrial setting, such as power plants and manufacturing plants, ungrounded electrical systems usually supply many field devices such as vital loads that must remain invulnerable to spurious trips such as certain plant control functions, valve actuators, emergency equipment etc. An electrical ground is an electrically conductive body, such as the earth, which maintains a zero potential (i.e., it is not positively nor negatively charged). An electrical connection to a ground carries current away from the circuit.
Occasionally, on an intentionally ungrounded circuit, a ground fault may occur. When this happens the ungrounded circuit (unlike an intentionally grounded circuit) is designed to continue to feed the load. Undesirable ground faults on ungrounded systems can result from many several different situations. For example, some of the major failures in electric equipment are caused by insulation breakdowns. The insulation is affected by aging, humidity, dust and environmental conditions, operational parameters and maintenance or clean up practices. Over time, the insulation can degrade, thereby exposing the conductors to other conducting materials, resulting in an unintentional grounding. Other types of failures that may introduce ground faults include circuit board failures, excess moisture, bad wire joints and sleeves, leaking batteries, accidental grounds caused during testing, component failure, etc.
Additionally, there may be multiple ground faults on either the positive circuit leg (“hot side” for AC) or negative circuit leg (“common side” for AC) of a DC electrical distribution system or on both circuit legs at the same time. A DC electrical distribution system may have multiple branch circuits and each branch circuit may have many components being fed by the electrical distribution system. Ground faults are typically classified in one of two ways, as a “hard” ground fault or as a “soft” ground fault. A hard ground fault is a ground fault that offers little or no resistance to current flow. A soft ground fault is a ground fault that offers at least more than minimal resistance to current flow. In most electrical distribution systems, any ground that results in a detectable current flow in any monitored component will cause the actuation of a ground fault detection device (typically located at the main distribution bus), thereby alerting the operators of the electrical distribution systems to the presence of the ground fault. Once detected, the ground fault should be located and eliminated. A typical ground fault detection circuit consists of a pair of center-tapped resistors and the use of such standard ground fault detectors in electrical distribution systems is well-known to those skilled in the art and, accordingly, is not discussed in greater detail herein.
The existence of a single ground fault in a given system, while significant, is not as problematic as the occurrence of a second ground fault on the same system. Should a second ground occur prior to the first ground fault being located and repaired, the electrical distribution system could be seriously compromised. In this situation, extreme fault current may develop on the electrical distribution system with a possible complete de-energization of vital circuit(s) resulting. This is why it is of utmost importance to detect, locate, and repair a first ground fault at the earliest possible time, and most preferably before a second ground fault occurs on the same electrical distribution system. With multiple simultaneous ground faults, unexpected shutdowns of electrical equipment may occur, with the sudden operational failure affecting not only the operation of the equipment connected to the electrical distribution system, but possibly halting critical and related production systems and possibly resulting in serious equipment damage, hazards to personnel, and extended electrical system outages with longer repair times.
As demonstrated by the discussion above, ground fault detection is a very important consideration in ungrounded electrical distribution systems. However, equally important and far more difficult is ground fault location. A ground detection system only senses that a ground fault exists somewhere on what may be a vast ungrounded electrical system while pinpointing the exact location of the ground fault is a task that remains to be accomplished.
Techniques currently exist that enable the detection of a ground fault on an ungrounded system. For example, ground fault detectors (e.g. ANSI device function number 64) are available that sense that a ground fault exists somewhere on a vast ungrounded electrical system. Such fixed ground-detecting equipment is typically used to detect and indicate the presence of a ground fault on a large distribution system. Once the ground is detected, an alarm will sound at the larger or higher-level distribution switchgear indicating a ground fault has occurred on the system. Some contemporary ground detection devices allow multiple alarm set points that are initiated at various levels, depending on factors such as the amount of impedance or resistance associated with the ground fault.
These devices will typically activate a primary alarm at a localized switchgear location when a ground fault reaches a specific magnitude. As the ground fault reaches a stronger magnitude, a secondary alarm will occur typically in an industrial plant's control room indicating a more urgent need to locate the fault. Once again, these cascading alarm systems are useful for indicating the presence of a ground fault without providing any location information that may be used to remedy or eliminate the ground fault. Thus, while techniques currently exist that will alert a user that a ground is somewhere on the system, it cannot accurately identify on which branch circuit the ground is located. In addition, depending on the amount of personnel located near the alarm system, a localized alarm may go unnoticed and the ground fault undetected until a second ground fault occurs, leaving vital systems at risk.
Techniques, independent of ground detectors, are also available to perform ground fault location, which is the act of finding the source of the ground. These various methods and techniques generally employ a standard current transformer and a method to vary the current flow in some fashion. However, a typical current transformer will only provide an output when monitoring a rising and falling current flow. Accordingly, on a DC circuit, a current transformer will not provide a measurable output unless the current is manipulated in some fashion.
Portable ground fault locating techniques used on uninterruptible systems supplying vital loads typically attempt to locate the circuit containing the ground fault by causing the ground fault current to vary in magnitude, thereby providing a signal that can be detected. A current transformer is then used as a detector to sense associated ground fault current changes, systematically on every circuit of the system, until the circuit containing the ground fault is located. While effective in certain limited circumstances, given the possibility of hundreds of circuits that may need to be checked, this technique can be very time-consuming and labor intensive. Additionally, there are a number of other limiting factors that make present techniques less than optimal.
For example, many high resistance ground faults have very low levels of current flowing in them, making the use of a standard current transformer practically useless in a ground fault detection scenario. If the ground fault current is very low, the current transformer will not have an output of any measurable size and the circuit with the ground fault cannot be located. Additionally, many circuits suffer from higher frequency “noise” in the circuit, with the possibility of eliciting a false positive result for most techniques used to locate ground faults. Additionally if an amount of normally detectable ground fault current is available but yet lower in magnitude than the noise on the electrical distribution system, traditional ground fault location equipment will not be able to distinguish between the actual ground fault and the ambient noise on the circuit and the result will be an inability to locate the circuit containing the ground fault. As explained herein, traditional ground fault locating equipment may be incapable of detecting high resistance ground faults due to either a low amount of ground fault current, excessive noise on the system, or a combination of both.
In these relatively common situations, the only other commonly known method employed to locate the circuit containing the soft ground fault is the “breaker isolation” method. In this method the technician must systematically open each branch circuit starting with the one offering the least risk to vital equipment, gradually moving up to ones of higher risk. This method is considered very undesirable because it can actually present a greater risk of inadvertent shutdown or equipment malfunction, based on loss of electrical power, than the actual ground fault itself.
In addition, since contemporary practices in the industry generally rely on a separate and unrelated detection devices and admittedly marginal location methods, they offer very limited opportunities in locating intermittent, cycling or momentary ground faults or multiple ground faults on an ungrounded system. An intermittent ground fault results from a ground fault occurring in electrical equipment during a specific operation but not in any specific time cycle. In an industrial setting, various types of equipment may be of a power cycling nature between on and off. If this equipment also contains a ground fault, the detector will only sense the fault when the equipment is in the “on” position but not in the “off” position. Typically, after a ground fault has been detected by the detection system, an alarm is actuated, and a technician is dispatched in an attempt to locate the source of the ground fault or ground faults. During the time that the technician is investigating the ground fault, the strength of the ground fault may change or the ground may become intermittent, cycling or momentary stopping altogether, consequently making the entire location effort futile. Other ground faults may be hidden in control circuit operations and may occur only during the transitory operation of a single switch. The detector may detect the ground fault during the switch manipulation but not sense it when the switch is released, thereby sensing a momentary ground fault that will not be located when personnel are dispatched to investigate.
Thus, while certain techniques are available for detecting and locating ground faults in electrical distribution systems, present systems and methods are sub-optimal due to the inherent limitations in both the equipment and techniques known to those skilled in the art. Accordingly, it would be an improvement in the art to augment or even replace current equipment and techniques for both ground fault detection and location.